The present invention relates to the art of earth boring and, more particularly, to a rolling cutter rotary rock bit having varied pitches between the teeth or inserts of the bit.
The rapidly increasing demand for the earth's natural resources such as oil and gas and various types of ores extracted by the mining industry has created the need for improved drilling bits. Rotary rock bits of the type embodying the present invention are adapted to be connected as the lowest member of a rotary drill string. As the drill string is rotated, the bit disintegrates the earth formations to form an earth borehole. Drill bit cutter life and efficiency are of prime importance in the drilling of such boreholes since the penetration rate is related to the condition of the bit and the operational efficiency.
Traditionally, rolling cutter rotary rock bits have three individual arms that extend angularly downward from the main body of the bit. The lower end of each arm is shaped to form a spindle or bearing pin. A rolling cone cutter is mounted upon each bearing pin and adapted to rotate thereon. Each of the cutters includes spaced circumferential rows of teeth or inserts offset in relation to the corresponding rows on the other cutters to drill the earth formations at the bottom of the borehole. The portions of the earth formations broken up by the cutting structure are removed from the borehole by a flushing drilling fluid such as drilling mud or air. Rolling cone cutter rotary rock bits in general can be categorized in two general classes. The first is tooth bits having generally chisel-shaped teeth integral with the body of the cone cutter. The second is insert or compact bits wherein individual inserts or compacts are press-fitted into holes in the cone body. The head of the insert projects from the cone cutter body and acts to break up the earth formations at the bottom of the borehole. The present invention can be employed in both classes of bits.
A rolling cutter rotary rock bit is designed so that the gage row inserts/teeth on the cutter determines the revolutions of the cutter with respect to revolutions of the bit. If the cutter was completely true rolling, the revolutions of the cutter would be equal to the circumference of the hole divided by the circumference of the cutter at the gage tip times the revolutions of the bit. The cutter would turn approximately 1.7 times the revolutions of the bit. However, the cutters are not designed for true rolling and the surface of the cutters have projecting inserts/teeth. The cutters will turn approximately 1.2 to 1.5 times the revolutions of the bit. The difference between true rolling revolutions and actual revolution of the cutter is the slippage or action on bottom.
Prior art bits have been restricted in their performance because of "tracking" and "stumbling." During the rotation of the rock bit each cone is "driven" by a row of heel (outer) inserts/teeth meshing with impressions which are cut into the bottom of the borehole by the combined heel inserts/teeth of all three cone cutters. When the relationship of the heel inserts/teeth on an individual cone cutter with respect to the combined heel impressions on the bottom of the hole is such that an inner row of inserts/teeth falls into its own previous impressions, tracking exists. Since an insert or tooth cannot dig effectively by hitting in its previous impressions, tracking is to be avoided. Stumbling is related to tracking in that the driving teeth on a cone cutter strike upon the flanks of rock teeth impressions previously laid down by the combined heel teeth pattern and skid, slide or "stumble" into the rock teeth impressions. The cause appears to be insufficient spread in the pitches of driving teeth numbers.
The "pitch" between rock bit inserts/teeth, refers to the measured, straight line, distance between centerlines, at the tips, of adjacent inserts/teeth. The pitch between inserts/teeth is useful in comparing different designs because a given pitch may cut satisfactorily in a formation of a certain hardness and/or abrasiveness and not cut satisfactorily in a formation of a different hardness. Also, pitches trend from wide to narrow as the design of bits trends from soft to hard formations. Within a given type bit, pitches trend with diameter of bits and diameter of inserts/teeth; i.e., a larger bit usually requires larger inserts/teeth diameters and lengths; and, consequently, due to required clearances between larger inserts/teeth, larger pitches are necessary. "Pitches" sometimes cause problems which are related to tracking and/or stumbling.
A cutter having evenly spaced inserts/teeth with wide pitch will cut a basic pattern in the bottom of the hole. The portion of the rock or formation between the cuts in the bottom are called rock teeth. With one cutter on the bit, these rock teeth will increase in size because the inserts/teeth on the cutter will try to fall or gear to the bottom. If the cutter was rolling true, it would gear to bottom and the bit would not drill. This condition is tracking. Since the cutter does not roll true, the inserts/teeth will not fall in track and the rock teeth will be removed. By having one row of inserts/teeth on each cutter, cutting the same track at the outer part of the hole, the rock teeth are removed or reduced in size; therefore, the bit will drill ahead. With two cutters and a smaller pitch between the gage inserts/teeth will try to fall in track with the coarse rock teeth.
Another problem encountered with prior art bits is inner row dominance. On occasion, due to combination of inserts/teeth numbers, instead of the outermost rows of inserts/teeth setting the driving pattern, an inner row, usually the second or third row from the hole wall, dominates the pattern and sets the driving pattern for that particular cone cutter. This driving pattern causes the outermost row of inserts/teeth to run in an abnormal pattern which exerts forces laterally upon the inserts/teeth and tends to cause breakage of inserts/teeth. The present invention tends to prevent inner row dominance and helps prevent outer row breakage.